This invention relates to coating deposition and plasma processing (ion implantation, etching, etc.) and particularly to magnetron cathodes.
Magnetron cathodes, in which a closed-loop magnetic field is established over at least part of the evaporable surface of the cathode, have come into wide use over the last 2 decades or so in the arts of sputtering and arc evaporation. In the case of a sputtering cathode the magnetic field serves to intensify an inert-gas plasma discharge and guide the plasma in a closed-loop path along the evaporable surface. In the case of an arc cathode, the magnetic field serves to guide the direction of one or more arc spots in a closed-loop path along the evaporable surface. Similar cathode and magnetic field geometries have been used for both sputtering and arc evaporation, with the main differences being the magnetic field strength required and the means of lateral confinement of the discharge. Sputtering cathodes have field strength of typically several hundred Gauss, while arc cathodes typically have field strength of only a few Gauss or tens of Gauss. Most conventional, currently used magnetron cathodes can be described as having basically planar or cylindrical geometry.
Planar magnetrons generally comprise a flat circular or rectangular plate of the material to be vaporized. A magnetic field is projected through or over the plate to form a closed-loop, magnetic tunnel or xe2x80x9cracetrackxe2x80x9d over the evaporable surface as disclosed for example in U.S. Pat. No. 5,407,551 (Sieck, et al.), U.S. Pat. No. 4,162,954 (Morrison), U.S. Pat. No. 4,673,477 (Ramalingam, et al.), and U.S. Pat. No. 4,724,058 (Morrison). The magnetic tunnel guides and contains the sputtering or arc discharge, typically forming a circular or oval erosion groove on the evaporable surface (the cathode surface from which material is vaporized). Material vaporized by either process is emitted in directions substantially perpendicular to the evaporable surface. Substantially perpendicular directions are understood for the purposes of this invention to refer to an emission distribution centered around the perpendicular to a surface, in which the amount of material emitted from a particular point on the cathode in a particular direction falls off as a function of the angle away from the perpendicular at that point. Substrates to be coated typically face the cathode surface and may be rotated and/or translated to extend the area of uniform coverage. Portions of the cathode surface may be inclined with respect to a planar surface, as disclosed in U.S. Pat. No. 4,428,816 (Class, et al.) and U.S. Pat. No. 4,457,825 (Lamont) in order to influence the distribution of emitted material or the cathode erosion profile.
A rectangular planar triode sputtering apparatus is disclosed in U.S. Pat. No. 4,404,077 (Fournier) in which a parallel field component extends over a non-closed path on the evaporable surface, with an electron emitter at one end of the path and a collector at the other end. A rectangular planar arc cathode is disclosed in U.S. Pat. No. 5,480,527 (Welty) in which the polarity of a parallel field component is reversed to make an arc scan back and forth along the length of the evaporable surface. A rectangular arc evaporation cathode is disclosed in U.S. Pat. No. 5,380,421 (Gorokhovsky) in which the evaporable surface is one side of a rectangular plate having beveled edges, and in which combined static and dynamic magnetic means are taught to control the arc movement along the length. A magnetron sputtering cathode is disclosed in U.S. Pat. No. 5,277,779 (Henshaw) comprising a rectangular frame, in which the erosion path wraps around the inner periphery of the frame, vaporized material is directed inwardly toward the center of the frame aperture, and substrates to be coated are passed through the aperture. A two-sided planar magnetron sputtering cathode is disclosed in U.S. Pat. No. 4,116,806 (Love) which has a separate closed-loop magnetic tunnel on each of two planar targets disposed on each side of a central frame comprising magnetic means. A planar magnetron cathode for either arc evaporation or sputtering is disclosed in U.S. Pat. No. 5,160,595 (Hauzer, et al.), in which part of the magnet means may be moved relative to the target surface in order to adjust the field strength depending on the vaporization method to be employed.
Cylindrical magnetrons generally comprise a cylindrical bar or tube of the material to be vaporized. The evaporable surface is generally the entire exterior or interior cylindrical surface, while the emission distribution depends on the particular magnetic configuration. A cylindrical sputtering cathode with a solenoidal magnetic field parallel to the long cylinder axis is disclosed in U.S. Pat. No. 4,031,424 (Penfold, et al.) which has an emission distribution perpendicular to the exterior surface and (ideally) uniform around the circumference and along the length. Sputtering and arc cathodes using magnetic means inside a cylindrical target to generate a closed-loop magnetic tunnel and erosion track over part of the exterior surface are disclosed for example in U.S. Pat. No. 4,417,968 (McKelvey), U.S. Pat. No. 5,364,518 (Hartig, et al.), and U.S. Pat. No. 4,849,088 (Veltrop, et al.), which employ relative movement between the magnet means and the target cylinder to achieve uniform erosion of the target. The magnetic means may remain fixed while the cylinder rotates or vice versa. The emission distribution is substantially perpendicular to the points on the cylindrical surface comprising the instantaneous location of the erosion track. Short cylindrical arc evaporation cathodes with solenoidal magnetic fields are disclosed in U.S. Pat. No. 4,492,845 (Kljuchko, et al.) and U.S. Pat. No. 5,518,597 (Storer, et al.). Long cylindrical arc evaporation cathodes generally require dynamic means to ensure uniform arc movement over the cathode length, as disclosed for example in U.S. Pat. No. 5,269,898 (Welty) and U.S. Pat. No. 5,451,308 (Sablev, et al). A cylindrical arc cathode in which an external coil applies a magnetic field perpendicular to the long axis of the cathode is disclosed in Soviet Inventor=s Certificate 711787. In this case the arc spots are described to be confined in the area in which the magnetic field lines are near perpendicular to the cathode surface, and it is specified that arc motion around the circumference is achieved by rotating the coil around the cathode. The magnetic field does not in this case comprise a closed-loop tunnel or path over the cathode surface.
Insulator means for preventing arc discharge spots from moving off an evaporable surface are disclosed in U.S. Pat. No 4,430,184 (Mularie). Magnetically permeable ring means for preventing arc spots from moving off an evaporable surface are disclosed in U.S. Pat. No. 4,448,659 (Morrison), U.S. Pat. No. 4,559,121 (Mularie), and U.S. Pat. No. 4,600,489 (Lefkow). Shielding and gap means for extinguishing arc spots which move off specified evaporable surfaces are disclosed in U.S. Pat. No. 3,793,179 and U.S. Pat. No. 3,783,231 (Sablev, et al.). Conductive ring means employing eddy currents for containing an arc discharge are disclosed in U.S. Pat. No. 5,387,326 (Buhl, et al.). Projecting side-wall means for containing a sputtering discharge are taught in U.S. Pat. No. 4,515,675 (Kieser, et al.), U.S. Pat. No. 4,933,064 (Geisler et al.), U.S. Pat. No. 5,133,850 (Kukla, et al.), U.S. Pat. No. 5,266,178 (Sichmann, et al.), and U.S. Pat. No. 5,597,459 (Altshuler) in which outward projections of the target, magnetic poles, or shielding at the sides of the evaporable surface serve to provide lateral confinement of the plasma.
U.S. Pat. No. 4,581,118 (Class, et al.) discloses a magnetron substrate support electrode having a book-shaped rectangular body, and a magnet core with flange-like pole pieces to provide a longitudinal magnetic field wrapped around the electrode body. The apparatus is taught to provide uniform plasma processing of a substrate mounted on the electrode, and is taught for use in conjunction with a separate sputtering cathode facing the support electrode and substrate. The substrate electrode is claimed to be connected to a power supply having voltage appropriate for ionization of the reactant gas adjacent to the substrate surface without causing significant sputtering from the substrate. The apparatus has therefore neither an evaporable surface nor a vapor emission distribution.
It is known to use arc evaporation and sputtering cathodes in ion or plasma sources for implantation or etching processes as disclosed in U.S. Pat. No. 4,994,164 (Bernardet, et al.), U.S. Pat. No. 5,404,017 (Inuishi et al.), U.S. Pat. No. 5,482,611 (Helmer, et al.). It is known to use ions from an arc evaporation cathode to sputter material from a biased secondary cathode for deposition onto a substrate. It is known to use arc evaporation cathodes in conjunction with CVD processes as disclosed in U.S. Pat. No. 4,749,587 (Bergmann) and U.S. Pat. No. 5,587,207 (Gorokhovsky). General descriptions of sputtering and arc evaporation equipment and processes may be found in xe2x80x9cThin Film Processesxe2x80x9d by J. Vossen et al. (Academic Press, 1991), xe2x80x9cHandbook of Vacuum Arc Science and Technologyxe2x80x9d by R. Boxman et al (Noyes, 1995), xe2x80x9cGlow Discharge Processesxe2x80x9d by B. Chapman (Wiley, 1980) and xe2x80x9cThin Film Depositionxe2x80x94Principles and Practicexe2x80x9d by D. Smith (McGraw-Hill, 1995).
Sputtering cathodes having the shape of a bar of substantially rectangular cross-section, having erosion surfaces wrapping around a lengthwise periphery of the bar and having substantially bidirectional deposition distributions are disclosed in U.S. Pat. No. 4,194,962 (Chambers et al. 1980), U.S. Pat. No. 4,486,289 (Parsons et al. 1984) and U.S. Pat. No. 4,812,217 (George et al. 1989).
A magnetron cathode is disclosed herein which has a different shape, magnetic field geometry, and emission distribution than conventional and currently available magnetron cathodes. In the present invention, the cathode has the shape of a rectangular bar (parallelepiped) as shown in FIG. 1. Erosion of the cathode material occurs from an evaporable surface wrapping around the periphery of the bar, along two opposite sides and around both ends. The vaporized material emitted from the evaporable surface is therefore distributed mainly in two opposite directions perpendicular to the long axis of the cathode. Vaporized material is also emitted perpendicular to the ends of the cathode, however for sufficiently long cathodes the amount of material emitted in these directions is a small fraction of the total. The invention provides uniform emission over long cathodes, facilitating the coating or implantation of large substrates. Uniform erosion over long arc evaporation cathodes is accomplished without need for complicated switching or dynamic control schemes. Since vapor is emitted in two directions perpendicular to the cathode length rather than only one, the present invention also provides larger area coverage than a conventional rectangular planar magnetron of the same length. Cathode cross-sectional dimensions up to at least 10 cm and lengths up to at least 3 meters are practical according to the present invention, permitting long cathode operating life and large coating area coverage by current industrial standards.
A magnetic field is established around the entire periphery of the cathode using permanent magnets or electromagnets. The field has a component over the entire evaporable surface which is parallel to the surface and perpendicular to the long axis of the cathode. In the cases of both sputtering and arc discharges, the emitted secondary electrons or arc spots (respectively) are caused to move along the evaporable surface in a direction perpendicular to this parallel magnetic field component. Since the parallel magnetic field component is continuous around the periphery of a cathode of the present invention, the electrons or arc spots move around the evaporable surface in a continuous closed-loop path. Cathode material is vaporized from this erosion path by sputtering or arc evaporation, and emitted in directions substantially perpendicular to the surface. A field strength (flux density) for the parallel magnetic field component in the range of 5 to 50 Gauss is generally suitable for arc evaporation cathodes, while a flux density of 200-400 Gauss is generally suitable for sputtering cathodes. Higher field strengths may be desirable in some cases using either technology, for example with materials (such as carbon or copper) having low arc velocities, or when it is desired to sputter at low gas pressures.
The motive force around the closed-loop erosion path, as discussed above, is due to the magnetic field component parallel to the evaporable surface and perpendicular to the cathode length. Lateral forces on the plasma discharge, i.e. in the directions across the width of the erosion path, are also generally necessary to achieve controlled vaporization of (only) the intended evaporable surface. In the case of a sputtering discharge it is desirable to prevent the plasma from diffusing away from the evaporable surface laterally along the magnetic field lines, thus reducing the sputtering rate. In the case of an arc evaporation discharge it is desirable to prevent the arc spots from moving laterally off the intended evaporation surface and onto other cathode or connector surfaces. Various means for lateral control may be employed within the scope of the present invention, depending on whether the cathode is to be used for sputtering or arc evaporation. Lateral control means for arc discharge spots may include for example magnetic means, insulator means, permeable ring means, conductive ring means, shielding means, or projecting side-wall means. Lateral control means for sputtering discharges may include for example magnetic means or projecting side-wall means. Preferred embodiments are described below in which lateral control means are chosen to provide uniform cathode erosion and high material utilization efficiency.
The cathode is typically mounted in a vacuum chamber along with substrates to be coated or implanted, and operated at pressures below 50 mTorr in either arc evaporation or sputtering configurations. Inert and/or reactive gasses such as argon, nitrogen, oxygen, methane, etc. may be introduced into the chamber during operation. During operation the cathode is typically connected to the negative output of a dc power supply, and the positive output of the power supply connected to an anode. The anode may be an electrically isolated structure inside the vacuum chamber, or may be the vacuum chamber itself and/or any interior shielding, etc. In the case of a sputtering cathode, the power supply may have relatively high voltage and low current output capability (e.g. 500 volts and 20 amperes), while for an arc cathode the power supply may have relatively high current and low voltage capability (e.g. 500 amperes and 20 volts). In the case of an arc evaporation cathode the discharge is typically initiated by a mechanical trigger, electrical spark, or laser pulse, while in the case of sputtering simple application of high voltage to the cathode is sufficient to initiate the discharge. Alternatively or in addition to a dc power supply, the cathode may be operated with ac or pulsed power supplies. The substrates to be coated or implanted may be electrically isolated from the cathode, anode, and chamber, and connected to the negative output of another power supply for purposes of increasing the energy of ion bombardment during deposition or implantation. Alternatively the substrates may remain at or near ground potential while the cathode is biased to a positive potential.
In an arc evaporation discharge there are, in addition to the emitted plasma, also molten droplets of cathode material ejected by the arc. These droplets, referred to as macroparticles, are ejected mainly at low angles to the cathode surface. A further advantage of the present invention as compared to cylindrical and planar arc cathodes of the prior art is that a substantial portion of these macroparticles can be blocked from reaching the substrate by an anode or shielding structure extending outward from the sides of the evaporation surface. For a narrow cathode, relatively short side shielding as shown in FIG. 5 provides substantial macroparticle reduction with minimal blockage of vaporized material. For example in a coating system having substrates arranged in a circle around the cathode as described below, an arc evaporation cathode of the present invention has been found experimentally to reduce the number of macroparticles imbedded in a zirconium nitride coating by at least a factor of 3 compared to a standard commercial cylindrical arc evaporation cathode of similar size.
Substrates to be coated or implanted may for example be mounted in a rotating circular array around the cathode and along its length, or on an array of spindles with compound xe2x80x9cplanetaryxe2x80x9d rotation as shown in FIG. 9. Emission of material from both sides of the cathode provides more uniform coverage around the substrate array than can be obtained using a single planar magnetron of the prior art. This can be advantageous, for example, in the case of reactive coating deposition, in which it is desirable for reaction conditions to be as uniform as possible around the substrate array in order to obtain uniform properties (such as color). Various other substrate arrangements will be apparent to those skilled in the art. For example in a system with linear substrate motion, the double-sided emission distribution of the present invention permits two parallel rows of substrates to be coated simultaneously, one on each side of the cathode as shown in FIG. 10.
One objective of the present invention is therefore to provide uniform erosion and vapor emission in two opposite directions over extended cathodes, permitting uniform deposition or ion implantation over large areas in a variety of substrate configurations. Further objectives are to permit operation as either a sputtering or arc evaporation cathode by appropriate choice of magnetic field strength and lateral confinement means, to eliminate any need for dynamic arc spot control, to reduce the number of macroparticles emitted by an arc evaporation cathode, and to achieve high cathode material utilization in either arc evaporation or sputtering configuration.